What is the best way to stream from my PC

You may want to choose a forum name rather than your email id in a public domain to protect yourself from spam.

The sound quality should not vary much, if at all, between the connection types, however the caveat is really the length of the cables. USB can suffer if the cable is too long, but this will result in drop out or stuttering rather than any particular sound quality degradation. I would choose the tidiest way or the most convenient.

Regarding high resolution files, high resolution files in themselves do not guarantee a higher quality than 16/44.1, in fact do not offer any more quality in audibility terms. 16/44 is more than adequate for the entire audible human hearing frequency range, so you can save yourself some space and stick to CD quality lossless as a maximum. I find that 256kbps VBR setting to be indistinguishable from the lossless files, but still archive in ALAC. The AAC files are supposedly better than mp3 on a like for like compression basis, so if you want to keep loosy files, the perhaps AAC would be the better choice. I doubt that there's much in it though.

One last thing, I'd change your user name.

Which streming device are you thinking of using? The now defunct Squeezbox would be able to do the full 24/192, though how much support is available for it im unsure of.

Regarding the connections/protocols:

Ethernet has solid data transfer up to 80 feet and has the highest bandwidth with 100 gigabits per second. 

USB has solid data transfer up to 16 feet, it has a low bandwidth compared to ethernet with 480 megabits per second. Also the USB bus is likely to be shared among several users. Even if you are plugged into different ports you are probably sharing the same controler as all of the other devices on the bus, so your device is sharing the USB bus bandwidth with all of the other devices. 

 I do recal an Apple Mac Mini that audio folk use as it has a dedicted USB port, the port is individually controlled that has its own power rail which keeps noise down compared to a shared bus.

Coaxial is for short runs only with grounding.

Toslink has the lowest bandwidth but offers ground isolatuin between products and noise immunity over long runs.

Megabits dude, highest Ethernet transfer rate currently available is 10Gigabits (to my knowledge) and I don't think there are any home devices available that are using it. 1Gb ethernet is available, plenty of switches and so forth that'll give you that, although whether you'll actually get that much out of them (or actually need it) is a different matter.

>any of the three ways of transferring the file to a DAC has any sound quality advantages over the others<

IMHO get a designed for purpose music streamer then you are indepedent of all the bother of getting a bit-perfect stream via a PC soundcard and you're not falling over a piece of wire between the PC and the hifi.